A vehicle may contain safety restraint actuators which are activated in response to a vehicle crash for purposes of mitigating occupant injury. Examples of such restraint actuators include air bags, seat belt pretensioners, and deployable knee bolsters. These vehicle crashes may occur over a wide range of directions relative to the longitudinal axis of the vehicle, and the effectiveness of some restraint actuators can be directionally dependent. A particular air bag restraint system may provide the best occupant protection for collisions in one particular direction along the restraint system axis, and diminishing levels of protection as collision angles depart from the preferred direction. For example, a frontal air bag system provides the best protection for collisions which are directed along the longitudinal axis of the vehicle, while also providing protection but perhaps at a lesser degree for angular or offset crashes, with collision angle measured relative to the longitudinal axis of the vehicle. For collision angles less than 45 degrees in magnitude, the crash is primarily front directed, while for collision angles between 45 and 135 degrees in magnitude, the crash is primarily side directed, while for crashes between 135 and 180 degrees in magnitude, the crash is primarily rear directed.
Both frontal and side-impact air bag systems are well known in the art, and each system is preferably only activated for collisions within its respective range of collision angles. For example, a frontal air bag system might preferably not be activated during a side impact, and a side-impact air bag system might preferably not be activated during a frontal impact. Each such system would have an associated range of angles for which the system is preferentially deployed in the event of a crash for which the occupant might otherwise be injured.
The particular safety restraint actuator(s) which are preferably activated for a given range of crashes is referred to herein as a safety restraint system, whereby a given vehicle may contain a plurality of such safety restraint systems. For each safety restraint system in a given vehicle there is an associated set of crashes of various severity levels which are so directed as to require the activation of the safety restraint system in order to mitigate occupant injury. For the remaining crashes, the restraint system is preferentially not activated so as to minimize the risk of restraint induced injury to the occupant or to avoid unnecessary repair costs associated with the activation of the restraint system.
A safety restraint system is activated by a crash discrimination system which senses the acceleration associated with the crash and determines from the acceleration-time waveform if and when to send an activation signal to the safety restraint system. For an air bag system, this activation signal generally comprises a current of sufficient magnitude and duration to initiate an ignitor which in turn ignites the gas generant composition in an inflator to generate the gases necessary to fill the air bag. The crass discrimination system generally has a restraint sensing axis aligned with the associated restraint system axis. For example, for a frontal air bag system, the restraint system axis and the restraint sensing axis are both aligned with the longitudinal axis of the vehicle, whereas for a side-impact air bag system, the both the restraint system axis and the restraint sensing axis are perpendicular to the longitudinal axis of the vehicle. Generally, acceleration components directed along the restraint sensing axis determine the activation of the associated restraint system, although off-axis components of acceleration can sometimes be interpreted as axial components, especially if the sensor associated with the crash discrimination system is rotated in the course of the crash because of structural deformation of the vehicle.
A crash discrimination system must discriminate between crash conditions requiring restraint system activation--"ON" conditions,--and crash conditions for which the restraint system is preferentially not activated--"OFF" conditions. The borderline between these two conditions is referred as a threshold condition. Those crash conditions near the threshold for which the restraint system is preferentially not activated are referred as "threshold-OFF" conditions (e.g. 8 MPH), while those crash conditions near the threshold for which the restraint system is preferentially activated are referred as "threshold-ON" conditions.
One set of known crash discrimination systems utilizes a plurality of mechanical discrimination sensors positioned and mounted in various locations within the vehicle crush zone or the engine compartment. Each mechanical discrimination sensor generally has a characteristic damping level, which when increased, or over damped, causes the sensor to behave more like a delta-velocity switch; which when decreased, or under damped, causes the sensor to behave more like an acceleration switch. In order to prevent borderline crashes, i.e. "threshold-OFF" conditions, from activating the safety restraint system, mechanical discrimination sensors are generally overdamped, having a delta-velocity threshold in the range of 10-12 MPH, so as to prevent "threshold-OFF" conditions from activating the safety restraint system but with the associated disadvantage that the corresponding "threshold-ON" performance is variable. Generally mechanical discrimination sensors operate by closing a mechanical switch in response to an acceleration signal. U.S. Pat. No. 4,166,641 teach the combination of a crash sensor mounted in the passenger compartment with a front crash sensor to improve immunity to false activation of the front crash sensor.
In operation, any one of the plurality of mechanical discrimination sensors can activate the associated safety restraint system. Also, a safing sensor is generally placed in series with the safety restraint system to improve the noise immunity of the system, whereby to activate the safety restraint system, both any one of the mechanical discriminating sensors must be ON, and the safing sensor must be ON, where ON refers to the condition where the sensing characteristic of the sensor has exceeded its associated threshold level. In other words, the activation of the safety restraint system is given by the logical AND combination of the safing sensor with the logical OR combination of the plurality of mechanical discrimination sensors. Safing sensors typically are simply acceleration switches with a relatively low switching threshold (e.g. 1-2 G's) which is not suitable for crash discrimination because occupants could be harmed by the deployment of an air bag restraint system which might not otherwise be needed to mitigate occupant injury.
The prior art teaches mechanical discrimination sensors which are self-testable. U.S. Pat. Nos. 4,827,091, 4,922,065 and 5,430,334 teach the application of a current to an electromagnetic coil surrounding a magnetic sensing element to move the sensing element--which would otherwise move in response to a crash induced acceleration--thereby closing the mechanical switch contacts of the sensor. U.S. Pat. No. 5,485,041 teaches the use of a Hall effect or weigand wire sensor instead of mechanical switch contacts. U.S. Pat. No. 5,003,190 teaches a self-testable relay-like crash sensor. Such a self-test determines the diagnostic state of the sensor, which is either operative or inoperative. An operative sensor can be expected to properly discriminate crashes according to whether or not the safety restraint system should be activated, whereas an inoperative sensor would not be expected to provide such discrimination.
U.S. Pat. Nos. 4,827,091, 4,922,065, 5,430,334 and 5,485,041 also teach that a mechanical discrimination sensor can be reconfigured to assume a variety of different sensing characteristics by controlling the current to the electromagnetic coil surrounding or in proximity to the magnetic sensing element. For example, a safing sensor so constructed can be reconfigured to become a crash discrimination sensor. U.S. Pat. No. 5,085,464 teaches an air bag firing circuit incorporating a plurality of self-testable sensors whereby a faulty sensor is reconfigured to the open position. U.S. Pat. No. 4,958,851 teaches an air bag firing circuit incorporating first and second testable and reconfigurable crash sensors, whereby in the event of a malfunction of a crash sensor, the malfunctioning sensor is activated and if the malfunctioning sensor normally has the higher detection threshold the other crash sensor is reconfigured to have the higher detection threshold. U.S. Pat. No. 3,780,314 teaches another type of reconfigurable electromagnetic sensor, whereby the activation of first crash sensor mounted at the front of the vehicle causes the activation threshold of distinctly located second crash sensor to be lowered.
Another form of a mechanical discrimination sensor known in the art as a crush zone sensor generates a signal responsive to vehicle crush caused by the vehicle crash. These sensors are located within the crush zone of the vehicle structure associated with the set of vehicle crashes for which the safety restraint system is preferably activated. Examples of the principles by which such sensors operate include but are not limited to simple mechanical switch closure, fiber optic sensing, acceleration sensing, and magneto-restrictive sensing. U.S. Pat. No. 3,889,232 teaches the combination of a crush zone sensor with a mechanical deceleration sensor for actuating an air bag system.
Another known crash discrimination system utilizes a single point discriminating crash sensor comprising an electronic control module incorporating an accelerometer, whereby the electronic control module processes the acceleration waveform measured by the accelerometer and outputs a signal to activate the safety restraint system if selected properties of the acceleration waveform according to a sensing characteristic exceed a specific switching threshold. The sensing characteristic is typically implemented by an algorithm executed by a microprocessor in the electronic control module. This activation signal may take a variety of forms, including but not limited to a voltage level, a current level, or a switch closure. The single point discriminating crash sensor is generally mounted at a location within the vehicle from which an acceleration signal is observable for each crash within the set of crashes for which the associated restraint system should be activated. Examples of single point crash discrimination systems are found in U.S. Pat. Nos. 5,067,745, 5,365,114, 5,396,424, 5,495,414 and 5,587,906.
The prior art teaches accelerometer based crash sensors which are self-testable. U.S. Pat. Nos. 5,387,819, 5,506,454, 5,433,101 and 5,495,414 teach the use accelerometers which sense the capacitance of a moveable electrode, whereby the sensing elements may be self-tested with electrostatic deflection. U.S. Pat. Nos. 4,950,914 and 5,428,340 teach the use of piezoelectric sensing elements which are tested by use of a counter piezoelectric effect. U.S. Pat. Nos. 5,375,468 and 5,377,523 teach the use of a piezoelectric accelerometer coupled to a vibrator. U.S. Pat. No. 4,950,915 teaches the use of a piezoelectric sensing element which is tested with acoustic energy. U.S. Pat. No. 5,440,913 teaches the use of dual accelerometers which are continuously tested under normal driving conditions. U.S. Pat. Nos. 5,182,459 and 5,363,303 teaches the use of dual piezoelectric accelerometers which are each testable. U.S. Pat. Nos. 5,389,822 and 5,083,276 teach the AND combination of two acceleration sensors installed at approximately the same location. U.S. Pat. No. 5,422,965 teaches the use of plural self diagnosis algorithms to improve reliability.
Furthermore, the single point discriminating crash sensor may incorporate a safing sensor for improved reliability. U.S. Pat. No. 5,261,694 teaches that the safing sensor can be reconfigured as a crash discriminating sensor in the event that the single point discriminating crash sensor otherwise fails, whereby this reconfigurable safing sensor is co-located in a common housing with the accelerometer based discriminating crash sensor. U.S. Pat. No. 5,416,360 teaches the combination of a mechanical crash sensor with an electronic crash sensor for improved reliability. U.S. Pat. Nos. 5,338,062 and 5,428,534 teach the combination of a centrally located electronic crash sensor with lateral deformation sensors for purposes of detecting side impacts for improved crash detection and discrimination.
The prior art also teaches the use the a plurality of crash sensing algorithms in a single crash sensor, and also the use of an adaptive or variable threshold level. U.S. Pat. Nos. 4,994,972, 5,040,118 and 5,229,943 teach a plurality of algorithms which are simultaneously evaluated to provide improved crash detection. U.S. Patent teaches a plurality of algorithms which are evaluated alternately by a single CPU. U.S. Pat. Nos. 5,081,587, 5,262,949 and 5,407,228 teach a variable threshold level depending upon vehicle speed or deceleration level. U.S. Pat. No. 5,225,985 teaches the use of an adaptive reference level for discriminating the occurrence of a collision.
One advantage of the single point discriminating crash sensor in comparison with a plurality of mechanical discrimination sensors is reduced cost and simplified system configuration. However, for robust performance a single point discriminating crash sensor with an inferior sensing algorithm or mounted in an inferior location within the vehicle may require one or more redundant or auxiliary discriminating crash sensors in order to detect crashes that the algorithm otherwise would miss for which the safety restraint system is preferably deployed. A crash discrimination system incorporating such a plurality of discriminating crash sensors would then activate the safety restraint system if any one of the discriminating crash sensors detected the occurrence of crash for which the safety restraint system is preferably deployed. In other words, the signal for activating the safety restraint system is formed as the logical OR combination of the outputs from each of the constituent discriminating crash sensors.
The prior art teaches the control of activation of an air bag system on the basis of collision direction. U.S. Pat. Nos. 5,390,951 and 5,609,358 disclose systems incorporating a combination of mechanical crash sensor and accelerometer based crash sensor for detecting collision direction and magnitude upon which decisions are made to either deploy or inhibit deployment of associated plural air bag systems. U.S. Pat. Nos. 4,836,024 and 5,173,614 teach a pair of accelerometers which are angularly displaced left and right of the vehicle longitudinal axis to improve the response characteristic and to determine the impact direction. U.S. Pat. Nos. 5,202,831, 5,234,228 and 5,620,203 teach a combination of longitudinal and lateral crash sensors for detecting crash direction.
The mounting location of the single point discriminating crash sensor is a factor affecting the design of the associated sensing algorithm. The single point discriminating crash sensor is preferably located where the sensor can observe the crash signal for which the safety restraint system is preferably activated without being susceptible to erroneous crash induced signals for which the safety restraint system is preferably not activated. For example, in a frontal air bag system the associated single point discriminating crash sensor might be located in the right or left cowl or a location proximate to side-impact crush and thereby become susceptible to a side-impact crash causing an acceleration along the restraint sensing axis of the single point discriminating crash sensor so as to cause an erroneous activation of the frontal air bag system. This could occur for example if the crash deforms the structure upon which the single point discriminating crash sensor is mounted. In some cases this deficiency can be ameliorated by modification of the associated sensing algorithm, but generally only at the expense of worsened crash discrimination performance as evidenced for example by increased detection time, also known in the art as time-to-fire (TTF). Also, the single point discriminating crash sensor can be subject to false activation by certain abuse events such as objects or debris which impact the vehicle in proximity to where the sensor is located.
Generally crash discrimination systems which incorporate only mechanical discrimination sensors are not sufficiently robust to properly sense the full range of frontal crashes. Vulnerably positioned stand-alone single point discriminating crash sensors are generally unable to meet performance requirements, such as detection time, while simultaneously preventing activation from crashes for which the safety restraint system is preferably not activated. The incorporation of redundant or auxiliary discriminating crash sensors does not necessary prevent erroneous activations caused by off-axis crashes because the restraint system activation signal is formed from the logical OR combination of the outputs from each of the constituent discriminating crash sensors, so that any one sensor of the collection vulnerable to false activation could falsely activate the safety restraint system.